1. Field of the Invention
The present invention relates to a laser power controller and a laser power control method suitable for recording data onto an optical disk by land/groove track recording such as a DVD-RAM (Digital Versatile Disk Random Access Memory), and an optical disk recording apparatus including the laser power controller.
2. Description of Related Art
A recording apparatus of an optical disk records data by way of applying laser light and controlling a laser power. Optimization of a laser power is an important factor that affects the recording quality. A laser emission power varies with temperature or the like, and it is necessary to control to maintain a constant power under such a condition.
A typical control method is to implement loop control through the input of an output from a controller to control a laser power to maintain a constant level according to an output signal from a front monitor to monitor a laser emission output, which is referred to hereinafter as an APC (Automatic Power Control) circuit, into a laser driver to drive the laser, thereby controlling the laser power to maintain a constant optimal level.
The APC circuit is typically composed of an analog circuit as shown in FIG. 5. The APC circuit 200 includes a laser driver 201 to control a laser diode (LD), a front monitor 202, a gain adjuster 203, a S/H (Sample/Hold) circuit 204, an amplifier 205, an LPF (Low Pass Filter) 206, and a DAC (Digital to Analog Converter) 207.
The APC circuit 200 implements loop control. Specifically, in the APC circuit 200, the gain adjuster 203 adjusts the gain of a monitoring result of the front monitor 202, the S/H circuit 204 holds the signal with the adjusted gain in synchronization with an S/H pulse, and it is controlled such that the signal value is equal to a set value that is input through the DAC 207. The operational amplifier 205 outputs a control value for the laser driver 1 through the LPF 206. The output bandwidth of the LPF 206 is a low time constant in order to prevent overpower upon power ON or due to noise, so that it takes a certain time until a laser output reaches a desired value.
However, it is necessary to reach a desired power instantaneously upon start of recording or the like. The above-described loop control takes too much time and fails to reach a desired power instantaneously.
Further, in a DVD-RAM disk to record data on both of a groove track and a land track, an optimal recording power for recording on a groove track and an optimal recording power for recording on a land track are different. It is therefore necessary to switch a recording power instantaneously at a switching point from a land portion to a groove portion and a switching point from a groove portion to a land portion. However, in the analog loop operation as in FIG. 5, the laser output cannot instantaneously reach a desired value.
To address this problem, there is a technique of setting a laser power using a microprocessor or the like as shown in FIG. 6 rather than using the analog loop operation. An APC circuit 300 of FIG. 6 includes a microprocessor 325 instead of the operational amplifier 205, the LPF 206 and the DAC 207 of FIG. 5. The microprocessor 325 includes a monitoring section 326 to monitor the output of the S/H circuit 204 and a laser power setting section 327 to generate a control value to set a laser power. In the APC circuit 300, the monitoring section 326 such as an A/D converter in the microprocessor 325 monitors a level from the front monitor 202, and the laser power setting section 327 detects a change in the level and makes correction as needed.
The control method using the microprocessor 325 corrects a laser power intermittently. Therefore, a larger monitoring interval causes less accurate power control. On the other hand, a smaller monitoring interval causes an overload on the microprocessor in return for a higher accuracy.
A laser power control method which aims to maintain a stable write quality on an optical disk upon stop and resume of recording during protection against buffer under-run is disclosed in Japanese Unexamined Patent Application Publication No. 2003-99935 (Yamamoto). Yamamoto teaches, for quick restoration of a laser power upon start of recording, a technique of starting a power output with an output value (fixed value) from a microprocessor and then shifting to loop control with a signal from a front monitor after stabilization.
FIG. 7 is a view showing a laser power controller 400 disclosed in Yamamoto. Upon start of recording, an analog SW 428 is at H, so that a given level that is set in a laser power setting section 427 of a microprocessor 425 is supplied to the laser driver 201. An emitted laser power then is output from the front monitor 202, and a recording power is extracted by the gain adjuster 203, the S/H circuit 204, and the LPF 206. A monitoring section 426 of a microprocessor 425 monitors an output of the LPF 206. After it detects that the output level becomes a constant level, the analog SW 428 is set to L so as to shift to the loop operation according to a signal from the front monitor 202. It is thereby possible to shoot the recording power up instantaneously upon start of recording and then maintain stable loop control after that.
The laser power control using the APC circuit taught by Yamamoto is effective for CD-R/RW, DVD-R/RW, or DVD+R/RW disks where recording mark M is recorded on a groove track GT only, and no recording is made on a land track LT where address information (e.g. LPP (Land Pre-Pit) in DVD-R/RW) is stored as shown in FIG. 8. It is thus effective for a case where no abrupt change in recording power occurs during recording.
On the other hand, in a DVD-RAM disk shown in FIG. 9, recording is made on both of a groove track and a land track. In a DVD-RAM, an address signal is recorded as PID (Physical ID) per sector using a method called CAPA (Complimentary Allocated Pit Addressing). A recording area (a land or a groove) between CAPAs is wobbled. It is possible to locate the position of the next CAPA by counting the wobble.
In such a DVD-RAM, an optimal recording power for recording on a groove track and an optimal recording power for recording on a land track are different. It is therefore necessary to switch a recording power instantaneously at a switching point from a land portion to a groove portion and a switching point from a groove portion to a land portion. However, because the loop operation by the LPF 206 with a slow time constant is in progress at those points, it is unable to change a recording power instantaneously. Further, the APC circuit disclosed in Yamamoto requires monitoring by the monitoring section 426 to detect the stabilization of an output of the LPF 206 and switching the control according to the monitoring result, which causes slower processing of the microprocessor.